Dual Charging

[bidkev]

I've gotten that used to isolating my electrics in the tinny that I've always charged my batteries one at a time with them isolated.

Now that I've got a glass boat, isolation isn't so much of an issue so my question is this:

If I have my isolator set at 2 (dual) does that mean that if I charge one battery the other is also receiving a charge? Would save a helluva lot of bother from keep changing 'em over if it did.

Probably pretty basic question but then again, I've got a pretty basic knowledge ;D

cheers

kev

To accomplish something, the first person you have to defeat is yourself.

[onerabbit]

Im no expert Kev , but if the batteries are on "duel" , then the charge should flow from one to the other to create a balance between the two.

Muzz

[pirate_pete]

I have a dual batteries set up by an Auto Electrician. I have a Polarised socket wired to one battery and leave Isolator switch on Dual, I have a float charger attached over long periods between use and it charges both ok.

[familyman]

Yeah kev thats right dual will charge both but take twice as long as it is charging two in parallel.I recently fitted a cd player to my boat and wired up the constant supply to the house battery so that after the boat has not been used the main start battery is still ok even if the house battery is dead flat.The switch can be left in the off position and the only thing that gets power is the stereo memory.
cheers jon

[finga64]

yep, it'll work Kingtin...no worries except if one battery is dead flat and the other is near full charged.
Then it may be an idea to charge one then the other to be on the safe side. I assume the 2 batteries are about the same size/ type and construction??
But if the bateries are roughly the same and about the same state of disharge go for it!! [smiley=2thumbsup.gif]
I really hope this doesn't turn into a dual batteries thread :'(
Cheers Scott :)

[bidkev]

Thanks for the replies fellers. Yeah Scott, they're both identical batteries fitted at the same time when I bought the rig. One has been used more than the other though.

Another question :D If battery 1 has a bigger charge than battery 2, and the one with the bigger charge is the one that the charger is connected to, then I assume that it will be fully charged before battery 2. Is it likely that the trckle charge will then kick in as the charger senses the full charge of battery 1 and battery 2 will only receive a trickle before it is fully charged? Or will the charger sense them in tandem and despite battery 1 being fully charged sense that both aren't fully charged and not go into trickle mode till battery 2 is fully charged?

cheers

kev

Time is what keeps everything from happening all at once.

[finga64]

Gidday Kingtin,
should sense the overall state of charge as the battery with more charge will help the flatter battery.
Does that make sense??
Cheers

[Burley_Boy]

The basic principle is quite simple. If the two batteries are simply hard wired to each other they will actually work at charging the least charged battery first.

Think of it like a pressure system with each battery being a gas bottle and connected in parallel while being filled.
If one is empty and the other nearly full then initially it all depends upon the available grunt & pressure of the filler (charger) if it can't keep up then the full one will help to fill the empty one. Basically they will try to reach a state of equilibrium so I think you will actually not start filling the almost full battery until the other battery has reached a similar state.

Quite easy to check actually.
Place a voltmeter each one of the batteries while in single mode (think of it like a pressure meter) switch into dual mode and watch the voltage on one of them drop, if the battery shows a higher voltage after you start charging and switching to dual then the battery is getting charged (assuming there is not a battery fault).
If however the battery shows a lower voltage than it did prior to switching to dual then its just helping charge the second battery.

Hope that makes sense.

[Manjilad]

Hi KingT,
Enjoy your posts … especially when you catch something!

I really wonder why my rig has a B1+B2 (dual battery) switch setting. For the life of me I cannot think of any reason why you would have such a setting?? … well only if you have 2 flat batteries and you desperately need a start …. BUT then 2 flat batteries defeats the whole purpose of have dual batteries ……. Maybe you should have 3!!!!
AND as you mentioned – using it for charging. Well I DON’T and here’s my reasoning ….

1. Connecting 2 batteries together in parallel will lead to instantaneous (high) current as the batteries race to the same voltage (charging one and discharging the other) – its that initial current flow I do not like - it generates heat, explosive gases and ages or even damages your batteries depending on the initial charge state of each battery. This is of course why, when you have a flat battery and need a start, you switch to the other battery- NOT dual batteries (unless you want to flatten your good battery!)
2. I charge each battery in isolation.
3. If I had a good quality automatically switching trickle charger (float charger) then AFTER my charging I would switch to the dual position and trickle charge both batteries say weekly until I next went out on the blue yonder! Now when I buy a quality float charger I have a reason for using DUAL position! Thank’s KingT you’ve taught me something again!
4. As an added precaution I am always careful to also ensure ALL my electronics is OFF and my isolator switch is OFF when charging because I do not want the the higher volage of a charger (and some spikes from modern “switching” chargers) getting at my electronics with the possibility (although unlikely) of damaging them (eg not to mention that my sounder and GPS “BYPASS” the isolation switch and are directly and permanently connected to the batteries)
By the way, charging on “dual” of course puts the charging voltage into ALL of your electronics ….
5. A thot for those who worry – what if you have a weak cell or worse a failed cell in one battery .. what would happen switch to "dual" or charging on "dual" ...?

To your latest post, the charger only sees a load which it assumes is one battery - be it one battery or 2 batteries and it does not know any different. So it will not switch to trickle until it senses the battery(ies) is/are near fully charged – here is the rub a cheap trickle charger may never switch to trickle if you have two batteries connected … a good way to overcharge and damage batteries … HENCE I will be buying a charger that can handle dual batteries!
Cheers
PS
A very helpful table of readings AT THE BATTERY for those who have not seen it.

STATE OF OPEN CIRCUIT SPECIFIC FREEZE Charge VOLTAGE GRAVITY POINT (Celsius)
100 % 12.60 1.265
90 % 12.57 1.245
80 % 12.54 1.230 -32
70 % 12.45 1.216
60 % 12.36 1.202
50 % 12.27 1.190 -24
40 % 12.18 1.175
30 % 12.09 1.160 -16
20 % 12.00 1.148
DEAD 11.70 1.120 - 8

TIP Take your multimeter reading under load (ie switch on a cabin light etc) to remove the "surface charge" overreading problem.

[bidkev]

Welll, I thought I had it sussed till now manjilad. ::) ;D Are you saying that I shouldn't charge in dual and also that I shouldn't charge unless isolated in case it damages the electrics, or am I misunderstanding?

If the latter were the case, then it would be standard practice to take your flat battery out of your car before charging, would it not?

I did notice that charging in dual for 36 hrs still showed my charger on full charge and not trickle. Normally, it only takes about 8 hrs to charge each battery, but it could be that my batteries were more discharged than they would normally be.

kev

Even if you are on the right track, you'll get run over if you just sit there!

[Manjilad]

Bluntly, YES! In order of Priority I would:
1. charge separately for safety and to squeeze maximum life from my batteries.
2. Only charge in dual mode for top up trickling when completed 1. above.
3. For the really fussy ones(like me!) - Yes there is a chance of damaging your electronics but you would be unlucky! So I ensure all equipment is off and batteries isolated as much as possible.

Also as a rule I go out fishing on one battery and return on the other, just to keep them reasonably balanced and cycled.

The problem is that most cheap chargers (say < $50 at a guess!) are just that and increase the possibility of overcharging as well as damaging other connected equipment especially if it is powered on or you power it on while charging. ... by the way I use a cheap charger myself- cost me $12 about 8 years ago!!!!!! and a big loud timer to remind me to switch it off!!!

A correctly set up car charging system (alternator) and even your boat alternator etc are a better design for sensing battery charge levels and not damaging connected and even operational electronics while charging.

To be sure I do not trust the lights on my charger to tell me the battery charge state, but measure (under load) the terminal voltages with my multimeter (or GPS) as mentioned in my post to check the true charge levels (ultimately a hydrometer is the proper way to measure battery charge but I would have to remove my batteries to use an hydrometer).

Anyhow just my point of view and current practice! I get my best charge landing fish!!!!!
Cheers

[Angla]

Looks like one of those posts again

Kev, you should know not to ask about two battery systems or have an opinion on the 2 foot moreton bay chop.

Angla

[onerabbit]

Kev,
We always go out on one battery, change over half way through the day, & come home on the other, that way I can be pretty sure they are both getting a good charge .

Muzz

[finga64]

# # Kev,
# # We always go out on one battery, change over half way through the day, & come home on the other, that way I can be pretty sure they are both getting a good charge .
# # Muzz
That's the best way. And use the boat more to keep the charge up. :)
That's my excuse and I'm sticking to it. ;)

[onerabbit]

# # Kev,
# # We always go out on one battery, change over half way through the day, & come home on the other, that way I can be pretty sure they are both getting a good charge .
# # Muzz
That's the best way. And use the boat more to keep the charge up. :)
That's my excuse and I'm sticking to it. #;)

Ideally twice a week, (BLODDY WEATHER), in reality once a month lately.

Muzz

[flyingfish]

Hi,

You would think, if the two batteries are at a different state of charge/cycle, the unit with the greatter charge may be overcharged and therefor damaged. This is due to the charger not being able to sense that one of the batteries is overcharged. :-/ :-/

[Burley_Boy]

Flying fish, I'll go back to my pressure analogy, overcharging is like overpressurising a gas cylinder. Unless the pressure (read voltage) is too high then there will be no overcharging.

As the charge in the battery increases the current flowing in will decrease.

The sensing you refer to is usually simply no more than the charger limiting the max current flow until you get to the point where the voltage of the battery is high enough to hit what you would refer to as a fully charged battery. If the charger is set up for the correct voltage thenthe charge current would just slow to a simple trickle charge at this point. A battery with the correct voltage on a charger will simply not overcharge. Not saying all chargers are set up right though!! #

If the voltage on the charger was set to be the same as the fully charged voltage (as shown in the earlier table) then in theory the charge would cease at this point. In reality the voltage is usually fractionally higher and a trickle charge happens.